The size and weight of wind turbines and the blades thereof have increased which means that the size and structural strength of the rotor hub have increased, and the main rotor shaft and other components of the nacelle have to be dimensioned according to the increased loads on the wind turbine. Today, the main rotor shaft form a large and heavy structure as it has to transfer the rotational torque generated by the wind loads to the drivetrain in the nacelle, e.g. via a gearbox. This makes the rotor shaft difficult to handle during the assembly process and often requires the use of two cranes.
The two main shaft bearings are normally mounted when the main rotor shaft is placed in a vertical position. The rotor shaft is normally rotated by attaching at least one strap to the generator end of the main shaft and at least one other strap to the rotor end when the shaft is resting on its support frame in horizontal position. The straps are then coupled to guide wires in the cranes so that one crane guides the generator end while the other crane guides the rotor end. The main rotor shaft is then lifted and rotated into a substantially vertical position before being placed on the floor or on a platform. The main rotor bearings are then mounted by guiding them onto the shaft from the generator end. The use of two cranes means that the main rotor shaft cannot be rotated into a right angle relative to the floor, thereby creating a dangerous situation for the workers as the weight of the main rotor shaft will cause the shaft to sway as the rotor end is brought into contact with the surface of the floor. This also increases the risk of damaging the main rotor shaft as the rotor end is placed on its edge before the end surface is brought into contact with the floor. The use of two cranes presents a time consuming process which requires at least two workers to operate the cranes.
This can be avoided by first lowering the shaft onto a set of support elements of wood. The straps on the rotor end are then removed and the shaft is lifted off the support elements which are then removed and the shaft is lowered onto the floor. However, these support elements can accidentally be pushed out or removed thereby causing the main rotor shaft to sway uncontrollably, or even tip over. The main rotor shaft may be placed in an area of loose gravel or the like. The small rocks may block the mounting holes in the rotor end and then have to be removed after assembly which in turn increases the assembly time.
The main rotor bearings may also be installed when the main rotor shaft is positioned in a horizontal position. However, this is a much more complicated process which requires a more accurate guidance of the bearings, in particular the main shaft bearing, when they are moved along the length of the shaft. This assembly method presents a significant problem as the main shaft bearing has to be guided past at least one of the contact surfaces of the support frame in order to be brought into position on the shaft.